Tiling : Unflatten the hierarchy of the geometry tree

[clementcolin]

Currently, the tiling methodology only make it possible to create a "flat" tree, using the position of each feature.

We may want to make it possible to create more depth (independently from the creation of LOD).

For example, we may want to put all biggest building in a tile, that would be loaded first. Or, in the same boudning box, split features depending on their geometry resolution, or number of triangle, so that the most complex object are loaded last.

It would also be usefull to associate a Geometric Error parameter automatically using the size of a tile (and/or its features)